A stator having concentrated windings has been widely used in motors or electric generators. In the concentrated-winding stator, one-phase winding is wound on each tooth of the stator. This stator has an advantage that a height of the winding (winding-end section) can be lowered, so that the motor can be downsized. The concentrated winding stator has a small value of a wire-wound resistor, so that a small copper loss generated by a wire-wound current and a wire-wound resistor is expected. The motor thus can work efficiently. Further, normal winding on a slot section, which can accommodate the winding, increases a space factor of the winding in the stator. The wire-wound resistor value can be thus lowered, which achieves an efficient motor. Normal winding is provided to a stator-member divided corresponding to every one tooth of a stator. Then a plurality of teeth of the stator are linked into an annular shape. This is one of constructing methods of the concentrated winding stator. A linking section of every tooth wound with a wire is welded each other and formed into an annular stator.
In general, a stator is formed by laminating a plurality of core sheets of which thickness ranges from 0.5 to 0.2 mm. The surface of this core sheet is electrically insulated in order to reduce eddy current.
Each one of the teeth is formed by welding several spots on borders between laminated layers at right angles with respect to a direction of laminating the core sheets. The structure of each tooth is further described with reference to FIG. 7A and FIG. 7B. FIG. 7A is a front view of a tooth of a stator. FIG. 7B is an axial sectional view of the tooth of the stator, and illustrates a status of welded laminations of core sheets.
In FIGS. 7A and 7B, tooth 60 of the stator is formed by laminating a plurality of core sheets 61, and winding 63 is wound concentrically on the lamination via insulator 62. An end face of outer rim and an end face of inner rim of respective core sheets 61 are independently welded at welding spots 64.
However, this welding damages electrical insulation at welding spots 64 between the core sheets laminated. At the damaged spots, eddy current increases, and the motor efficiency lowers. An inner rim of the stator faces the rotor, and magnetic flux changes intensely on the inner rim. Therefore, the damages of insulation increases eddy current substantially, which results in lowering the motor efficiency.
Another laminating method uses adhesive core-sheets as material for insulating and coating the core-sheets. Adhesive has been applied to the adhesive core-sheets. These core-sheets are punched to shape into cores, and laminated in line. Then the laminated core-sheets are pressurized, heated, and bonded to be integrated into one unit. In the case of pursuing a highly efficient motor, the punched core-sheets are often annealed in order to remove distortion produced by the punching, because the distortion adversely affects magnetic characteristics. An example of an annealing condition is this;
800° C. for one hour; then cooling the sheets gradually. However, the adhesive does not work at this annealing temperature, and the sheets cannot be properly annealed.
According to still another method of laminating the core-sheets, peaks and valleys provided to parts of the core-sheets are caulked. The caulked parts according to this method, however, adversely affects the magnetic characteristics, and lowers the motor efficiency. Further, in the case of a motor having a large axial measurement, i.e., a great thickness of lamination, deflection thicknesswise is produced due to resiliency at the caulked parts. This deflection is unfavorable to the motor, in particular, of which stator is formed by assembling a plurality of divided members.